The present invention pertains to techniques in fabricating electrochemical cell electrodes. In particular, it is methods of removing burrs or other unwanted projections from the edges of electrode substrates. Electrochemical cells, particularly secondary cells, often use discrete electrodes fabricated by deposition of electrochemically active materials onto conductive substrates. These substrates are typically in the form of a thin flexible sheet of solid or porous metal. The function of the substrate is to retain the electrode active material in a configuration having a maximized surface area and to act as a current collector. Electrodes made in this manner are assembled in various ways to form an electrochemical cell. A configuration predominantly used in popular commercial cell designs is commonly known as a jelly-roll assembly. In this configuration, positive and negative electrodes fabricated on thin sheet substrates are laid back-to-back with interposed nonconductive separators. The electrodes are then wound into a cylindrical shape. This "jelly-roll" assembly is then inserted into a canister, connected to terminals, and permeated with an electrolyte. The separators are typically made as thin as possible to maximize the active volume and capacity of the cells. As a result, a recurring problem in these assemblies is electrical shorting through the separator between adjacent positive and negative electrodes. One factor in this mode of failure is the existence of rough edges or burrs on the electrode substrates. During winding and compression of the jelly-roll assembly, these burrs are pressed into the separator and penetrate it to create a short to the facing electrode. Failure of the cell may result. This problem is particularly found in cells using porous metal substrates.
Electrodes in some cells use porous or foam nickel metal substrates. The porous metal is a thin three-dimensional matrix of interconnected filaments or fibers. Porosity is typically in the range of 90 to 95 percent. These structures are discussed in detail in U.S. Pat. Nos. 4,957,543 to Babjak et al. and 4,251,603 to Matsumoto el al. The benefit of the porous metal substrate is the ability to deposit and capture electrode active material within the substrate matrix. However, these substrates are typically cut from larger sheets of stock porous metal. In the process of cutting, many of the metal filaments are cut between their interconnections resulting in filament points or burrs. The cut edges of the substrate have a multitude of these burrs. When these substrates are formed into a jelly-roll assembly, the filament burrs may easily penetrate the separators causing shorts. To prevent this, substrates must be deburred to remove the filament burrs back to a filament interconnection thereby eliminating the penetrating points. This must be done without destroying more matrix interconnections and in the process creating new burrs.
Prior methods of deburring porous metal substrates include low voltage electric deburring typical of deburring methods in other technologies. In those methods, a low voltage potential (15-20 volts direct) is created between the substrate edge and a contact surface. The substrate is pressed into contact with the surface, completing a deburring circuit. The relatively high current (15-30 amperes) removes the burrs by a combination of fusing and vaporizing the metal. By translating the substrate edge across the contact surface while maintaining contact, a fairly smooth edge may be produced. This method of contact deburring has significant disadvantages including sticking of the substrate edge to the contact surface. For this reason and others, contact deburring is not effective in a high volume production environment.
What is needed is a method of deburring porous metal substrates in which the filament burrs can be quickly and easily removed without damaging the substrate body or creating new burrs. Such a method should be easily integrated into current methods of a fabricating cell electrodes and electrode assemblies.